Basics of Infrared Photography

Infrared light

Infrared is the name for that part of the electromagnetic spectrum with wavelengths longer than the visible light. Infrared radiation is normally separated in three parts: Near Infrared (700-1200 nm), Mid Infrared (1200-3000 nm), and Thermal Infrared (3000nm-1mm). In IR photography, we only use Near Infrared. Digital camera CCD and CMOS sensors are sensitive to light with wavelengths between ~400 and ~1100 nm. This is from just in ultraviolet to the near infrared. Any converted CCD camera is therefore not sensitive to heat radiation. Of course heat sensitive devices exist, but these are base on entirely different techniques and do not use silica base sensors or glass lenses.

CCD sensors en Infrared:Typical sensitivity of standard CCDs
The sensitivity or a typical CCD sensor is limited by the internal IR and UV rejection filter of the camera. the shaded parts in the sketch show the response of each color band.
When an IR-pass fiter is used with a standard camera, the internal IR-block filter and the added IR-pass filter have exact opposite transmission and effectively block almost all light. With very long exposures it is still possible to obtain some results, but you will need to use a tripod and noise is often increased.
Sensitivity of converted CCDs
In a camera converted for IR-photography, the infrared rejection filter is removed and replaces by an internal IR-pass filter. This means you will unobscured vision trough the viewfinder and the sensitivity of the camera is close to standard. With a converted camera you can use short exposure times and handheld photography is easy.

Colours

IR is not visible to the human eye. To create visible images, we use the standard visible colours to display the luminosity of the infrared signal. This is an artificial way to create an image from an invisible part of the light spectrum. real colour therefore do not exist in IR photography and I will use luminosity when talking about the signal strength of IR light. Of course it is possible to use colours in the final images, but these are created in post processing and therefore have no reality value.

When we see colours in unedited IR photos, this is caused by the RGB colour filters at the sensor (the Bayer pattern). The RGB filter are not developed for IR, but each colour has a slightly different transmissivity in IR. This results in differences in luminosity that are interpreted by the in-camera software according to the standard colour pattern (Red\Green\Blue) and therefore result in coloured photos. In post processing these colours can be use in many ways to create contrast and colour in the photos.

IR light sources

Sun: Very suitable for IR photography, Energy in IR is as high as in visible light.

Shade\diffuse light: less energy available in IR as in visible because of differential scattering in the athmosphere (bluesky effect) therefore more exposure compensation is needed.

Flash: Suitable, IR output of modern electrical strobes is as high as visible light

Fluorescent\LED: Unsuitable. these lights have very limited IR output because they are designed to save energy by emitting visible radiation only.